The invention relates to an inductive sensor for use in detecting the level of conductive, non-magnetic liquids, especially for use in a high pressure, high temperature environment, for example, of the type used in liquid-encapsulated Czochralski (LEC) growth of crystals such as gallium arsenide or indium phosphide.
Inductive sensors have been known for use in a number of inspection and monitoring applications. The basic principles of inductive techniques are well known. More specifically, an oscillating current flowing in a coil causes the field of one winding to add to the field of the next winding. The fields pulsate, in turn generating a pulsating electromagnetic field surrounding the coil. Placing the coil a nominal distance from a conductive or metal target induces a current flow on the surface and within the target. The induced current produces a secondary magnetic field that opposes and reduces the intensity of the original field, and changes in the impedance of the exciting coil can be analyzed to tell something about the target or the distance from the target.
Examples of such inductive sensor systems are, for example, disclosed in "A General Method for Designing Low-Temperature Drift, High-Bandwidth, Variable-Reluctance Position Sensors" by R. L. Maresca; IEEE Transactions on Magnetics, Vol. Mag-22, No. 2, March 1986 and a brochure published by Kaman Instrumentation Corporation in 1982, application note number 108 "General Application Considerations Inductive Displacement Measuring Systems". These sensors, while generally working satisfactorily in detecting surface conditions of non-magnetic metallic objects in atmospheric conditions and the like, are generally not thought suitable for use in an environment such as Czochralski growth of crystals.
More specifically, in liquid encapsulated Czochralski growth (hereinafter LEC) of crystals, the environments are generally thought to be extremely hostile to such sensor systems in a manner such that detrimental effects of the environment on the sensor itself preclude reliable inductive measurements in such environments. It is often the case that long exposure to high temperatures will cause sensor measurements to drift despite the fact that there was no change in liquid level. Thus, accuracy is compromised. Further, in the growth of gallium arsenide especially, it is often the case that arsenic becomes deposited on portions of the coil of the sensor thereby shorting out the coil and making further position measurements unreliable. Thus, in the growth of such crystals it has generally been the practice to employ physical contact melt depth sensors.
A problem with physical probes in the field of crystal growth is that they tend to disrupt the surface of the melt. Typically, it is essential in the field of such crystal growth that conditions be maintained very stable inasmuch as such growth involves contacting a seed crystal to the melt and thereafter very delicately pulling the growing larger crystal being grown and pulled by the seed from the melt. Any disruptions in the surface of the melt can result in separation of the pulling seed crystal from the melt thereby disrupting and terminating the process of crystal growth. Another problem with electrical contact probes is the contamination of the melt with unwanted impurities in the semi-conductors melt from the contacts.
In accordance with the invention, these problems encountered by physical contact of crystal melt are avoided by providing an inductive type sensor system which can be employed in such hostile environment crystal growth techniques.